1. Field of the Invention
The present invention relates to a method of producing particles of polymerization products, particularly those having controlled particle sizes and size distributions, that are suitable for use in the powder making industry. Examples of the products include gap retainers, slip imparting agents, functional carriers, monodispersed particles having surface activity, standard particles, toners, and functional fillers that control the fluidity and gloss characteristics of paints. More particularly, the present invention relates to an improved process for producing particles having a size of 5-50 .mu.m by suspension polymerization.
2. Background of Related Art
The importance of powder making technology which takes advantage of the various functions of particles per se is increasingly recognized these days. Among the particles produced by this technology are gap retainers, slip imparting agents, functional carriers, monodispersed particles having surface activity, standard particles, toners, and functional fillers that control the fluidity and gloss characteristics of paints. In order to produce these particles by polymerization, emulsion polymerization is used most commonly today. In speciality applications, other methods of polymerization are employed, such as soap-free polymerization, dispersion polymerization, seed polymerization and swelling polymerization.
However, these methods of polymerization have several defects. For example, considerable difficulty is involved in removing non-negligible impurities such as emulsifiers. Second, the size of particles that can be produced is limited. Third, the production cost is exorbitant. Fourth, these methods are too complex to be suitable for large-scale production. Particles having a narrow size distribution are in most cases produced by emulsion polymerization but the size of particles that can be produced by this method is only about 1 .mu.m at maximum and producing larger particles is extremely difficult.
Suspension polymerization is also capable of producing particles, however, the only products obtained so far are nonuniform in particle size and have a broad particle size distribution. Since particle size and size distribution are closely related to the performance of polymer products in such aspects as mechanical strength, chemical resistance, color, transparency and moldability, improvements in those factors are desired. In suspension polymerization, the liquid droplets dispersed under agitation have various sizes and during dispersion they are subjected to repeated breadkup and coalescence to produce particles having an extremely broad size distribution. For this reason, it is very difficult to produce by suspension polymerization those particles which have as narrow a size distribution as monodispersed particles. Under these circumstances, one of the objectives in the powder making industry is to establish a simple technique of suspension polymerization that is capable of producing homogeneous particles.
The mechanism by which particles are produced by suspension polymerization is as follows. A disperse phase and a continuous phase are broken up by applied energy, such as agitation, to form droplets that are dispersed in the continuous phase. The droplets, if they are left as they are, are generally unstable and undergo repeated breadkup and coalescence, but eventually they are supplied with energy, such as heat, to be polymerized to form rigid and stable particles that are no longer capable of breadkup and coalescence. Therefore, in order to control the size of particles produced by suspension polymerization, one may control in some way the size of the droplets and the process of their breadkup and coalescence. In fact, however, there are many factors that relate to the size of the droplets, such as the characteristics of a disperser (which is hereinafter referred to as a "granulator"), its construction, shape, rotational speed, size, or the size and shape of the reaction vessel, the amount in which the reaction solution is charged, or the ratio between disperse and continuous phases in the reaction solution, its viscosity, as well as the type and amount of a dispersant used, and it is practically impossible to control all of these factors in a desired way. Therefore, in practice, some of these factors have to be fixed so that suitable conditions for producing desired particles are determined with the other factors being varied.
However, this approach depends so much upon a trial-and-error basis that it is not readily adaptable to design changes such as scale-up of the process. This is a serious obstacle to the production of desired particles and the process lacks flexibility, particularly for the purpose of producing particles that are to be used in powder form.
The present invention has been accomplished under these circumstances and an object of providing a method of suspension polymerization that is capable of producing smaller particles having a size distribution being controlled in an easy way.